Abstract Reactive oxygen species (ROS) are a significant byproduct of many intracellular and extracellular events. Cellular systems exist to limit ROS accumulation within cells; a loss of fidelity for these pathways over time is proposed to contribute towards the excessive intracellular ROS levels associated with a variety of disease pathologies. Protein folding is particularly susceptible to disruption upon elevated ROS, and a loss of folding homeostasis has also been linked to the accumulation of misfolded aggregates in various neurodegenerative disorders. We have uncovered two novel redox events that make use of ROS to alter the activity of molecular chaperones of the Hsp70 class during oxidative stress. We propose that these pathways normally serve to maintain folding homeostasis upon conditions of elevated intracellular ROS. Both events make use of sulfur- based post-translational events, but each system is unique in the means by which it influences chaperone activity and cell physiology. Our data suggest that both systems help to maintain normal cell function during overly oxidizing conditions. This proposal focuses on elucidating the mechanistic features of these two cellular redox systems. In Aim 1, we continue our characterization of the redox- signaling pathway we uncovered in the endoplasmic reticulum centered on a thiol-based redox switch in the Hsp70 BiP. We propose to address how adduct removal from BiP post-stress is achieved and regulated to maintain folding homeostasis. In Aim 2, we aim to describe the mechanistic details for a redox switch in the cytoplasmic Hsp70 co-chaperone Fes1. We will characterize how redox-dependent inactivation of Fes1 via methionine oxidation influences cytoplasmic Hsp70 function, and how inactivation of Fes1 is advantageous during overly oxidizing conditions. Successful completion of the proposed studies will provide insight into the basic cell functions used to manage cellular ROS and avert cellular damage.